Gas nitrocarburizing processing has been known as treatment for improving wear resistance of a part made of steel by forming a nitride layer in a surface layer portion of the part. More specifically, in the gas nitrocarburizing processing, a part made of steel is brought into contact with, for example, ammonia gas in a temperature range of less than or equal to an austenite transformation point of the steel, to form an iron nitride layer in a surface layer portion of the part. Since the nitride layer has an extremely high hardness, the gas nitrocarburizing processing has been widely used as heat treatment for improving wear resistance of parts.
The above gas nitrocarburizing processing is performed by placing a workpiece within a heat treatment furnace and heating the workpiece in an atmosphere containing ammonia gas. There have been known methods such as a method of introducing ammonia gas only into a heat treatment furnace as a heat treatment gas for forming an atmosphere (see, for example, Taizo Hara, “Design and Facts of Heat Treatment Furnace”, Shin-Nihon Casting & Forging Press, March 1998, pp. 185 to 188 (Non Patent Document 1)), a method of adopting a heat treatment gas prepared by adding ammonia gas to nitrogen gas as a base gas, a method of adopting a heat treatment gas prepared by adding ammonia gas to an endothermic converted gas as a base gas, and the like (see, for example, Japanese Patent Laying-Open No. 2002-69609 (Patent Document 1) and Japanese Patent Laying-Open No. 58-174572 (Patent Document 2)).